Semi-transparent organic solar cells (ST-OSCs) require carefully selected active layer materials and one key requirement, the average visible transmittance (AVT), can be optimised through an engineering strategy by choosing appropriate donors and acceptors. Herein, an efficient ternary active layer is fabricated by using two wide bandgap (3.0 eV) star-shaped small molecules BFN or BFSN and a middle bandgap polymer PM6 as mixed donors, and a narrow bandgap non-fullerene Y6 as acceptor. By controlling the ratio of BFN or BFSN and PM6, the AVT of films can be optimised without changing the thickness. Without optical engineering, compared to an AVT of 26% in the binary active layer (PM6 : Y6 = 1.3 : 1.5), the ternary active layers with BFN/BFSN : PM6 : Y6 = 0.65 : 0.65 : 1.5 display a higher AVT of 60% and 62%, respectively, at a thickness of 100 nm. By further increasing the ratio of BFN/BFSN in the active layer, the AVT of ternary active films based on BFN/BFSN : PM6 : Y6 = 1 : 0.3 : 1.5 at 100 nm thickness can reach 67%. The ternary ST-OSC based on BFN and BFSN in a ratio of 0.65 provides a comparable power conversion efficiency (PCE) of 10.01% (BFN) and 11.18% (BFSN) to 12.81% for the PM6:Y6 binary OSCs after a silver electrode deposition. However, in a higher blended ratio of BFN or BFSN (BFN/BFSN : PM6 : Y6 = 1 : 0.3 : 1.5), the PCE shows a significant decrease due to the morphology of the active layer, which shows that the components are less well mixed. It was also noted that BFSN-based ternary ST-OSCs offer higher PCE than BFN-based ST-OSCs because of higher hole mobility for BFSN compared to BFN.